The evaluation and assessment of gait, mobility, and activity have become increasingly important to many areas of rehabilitation as the desire and need for evidence-based healthcare continues to grow. Although functional evaluations often rely upon subjective assessments by a clinician or patient self-reports, objective measures have become increasingly preferred for their substantive evidence. Objective evaluation of patient mobility and function, or outcome assessment, is a critical tool for the appraisal of rehabilitation or training programs, justification for an intervention or medical device, or the evaluation of patient activity.
The term “activities of daily living” (ADL) refers to basic functions of everyday life that include such tasks as bathing, dressing, eating, and transferring from one place to another. For most individuals, however, daily activities include a variety of ambulatory tasks such as walking on level ground, maneuvering around corners, moving across inclined or uneven surfaces, and ascending or descending stairs. Participating in a sporting activity can add many other activities such as jumping, running, etc. In this context, true activity is a complex combination of many different biomechanical patterns, occurring randomly throughout the day.
The overwhelming majority of research pertaining to human locomotion has traditionally been conducted in the controlled confines of a gait laboratory, often while subjects move across a level surface, in a straight path. Gait analysis commonly includes the study of kinetics, i.e., forces and moments. This type of study is generally accomplished through the use of a force platform, which is a plate rigidly mounted in the floor of a laboratory for use in measuring applied forces and moments. Computational models enable researchers to extrapolate applied external forces to predict the internal skeletal forces that occur during a gait. While this information has provided researchers with a wealth of information pertaining to how the human body moves through space and how the musculoskeletal structure interacts with the changing momentum of gait, level walking in the laboratory environment encompasses only a small portion of a person's true ADL.
Measuring Real-World Activity
Incline walking has been explored as an alternative to level-ground walking in order to assess the body's response to uneven ground. Evaluation of the kinetic response in incline walking often requires modified instrumentation or unique assessment tools. Researchers have studied kinetics during inclined gait by accurately placing a ramped surface on top of two force platforms. Mathematical modeling was used to resolve the reaction forces and moments on the ramp surface based on the output signals from the platforms. Such a system is an effective laboratory tool, but the size and location of the inclined surface is limited by the placement of the force platforms. In a related recent study, researchers mounted a force platform into an inclined surface to evaluate incline gait dynamics in able-bodied subjects. This solution avoided the need to resolve forces from multiple platforms, but limited observation and analysis to a single step by a subject on the force platform.
Similarly, stair ascent and descent have been researched as an alternative form of gait. Like incline walking, stair kinetics are most often assessed through the placement of strain-gage-based systems, i.e., force plates disposed in stair steps. Such instrumentation has limitations similar to instrumented walkways, since only select steps can be measured and only within the laboratory environment. While such techniques provide effective control of confounding factors, these types of gait may not be truly representative of real-world activity.
Turning gait has also received attention as a potential measure of real-world activity. To date, clinical assessment of turning gait has been derived from qualitative measures or single-step analysis in a gait laboratory. Recently, it has been demonstrated that a rate gyroscope, tethered to a laptop computer, can be used to detect and measure the rate of turning during ambulation in able-bodied subjects. Turns were categorized as soft or corner turns and were identified by a gyroscope voltage output of more than two standard deviations relative to the mean straight-walking reference signal. While such a device holds potential for identifying maneuvering motions in gait outside of a laboratory, the restrictions of a physical tether coupling the device to a computer and the discrimination between only straight or turning gait limit the application of this type of device to only a portion of the true activity domain.
The desire to measure kinetic data during real-world activity with portable instrumentation has been a long standing goal in the biomechanics community. One measurement system that has successfully evolved to record real-world activities is the plantar-pressure measurement system. In this system, thin membrane-like sensors are placed in the insoles of a subject's shoes. Pressure sensors monitor and record the foot plantar pressures during ambulation. Originally, such systems were used to measure only vertical force, were expensive, possessed limited sampling time, and again required direct cabling to a computer. Later systems of this type offered wireless data collection and improved sampling rates, but could acquire continuous data for a maximum time of only a few minutes. Recent commercial systems, such as the Pedar Mobile-X Novel™ (Munich, Germany) offer Bluetooth wireless data collection, have run times of several hours, provide on-board data storage, and exhibit enhanced resolution. Other systems include the Paromed™ (Neubeuern, Germany), Tekscan F-scan™ (Boston, Mass.), and TrueLife SmartFoot™ (Poulsbo, Wash.). Recent studies with these wireless devices have explored the ability to use pressure insoles to resolve the ground reaction force (GRF) by analyzing the pressure data. The estimation required to calculate the anterior-posterior force, especially in periods of double-support, is noted by researchers as a source of error. Additionally, pressure insoles are susceptible to drift, which can cause errors of up to 34%.
Step activity, or the number of steps taken over a given period of time, can be used as an alternative to directly measured forces and moments as an indicator of overall activity. Pedometers and step activity monitors are devices used to measure step activity during ambulation. Research has shown that many pedometers are highly accurate (>90%) for speeds above 3 mph, but become less accurate (<70%) at speeds below 2 mph. For elderly persons or patients with a pathology that can decrease self-selected walking velocity such as a lower-extremity amputation, such devices may not be appropriate. Enhanced step monitors, like the Cyma (now Orthocare) StepWatch™ (Mountlake Terrace, Wash.) use uni-axial accelerometers and signal processing to refine the accuracy of step counts. Software settings enable customization of each monitor to a patient, achieving an accuracy of, for example, about 96% (stairs) to about 99% (level walking). Step activity monitors have been used successfully to measure activity in diabetic patients and amputee subjects. However, to date, there has been only limited step activity research applied to evaluate accelerometer-based devices when comparing different types of activities, such as stair ascent or walking on uneven terrain.
Although instrumented stairs/inclines, rate gyroscopes, insole pressure transducers, and accelerometers each offer the potential to reveal a portion of the activity domain, none offer the ability to measure, evaluate, and classify multiple gait activities, which is particularly important for patients with compromised activity levels. One patient group where this limitation is clearly apparent is the amputee population. Loss of a limb can greatly influence the biomechanics of gait and the energy expenditure required for ambulation. For such patients, navigating the terrain of the real world can be challenging. The prosthetic industry has responded to the need of amputees by developing products designed to improve function across a variety of activity domains, including stairs, inclines, uneven terrain, and turns. Energy storage and return (ESAR) feet, microprocessor-controlled knees, shock-absorbing pylons, and rotation adapters are all examples of components designed to accommodate real-world activity domains. Unfortunately, the need for and the function of these devices is limited by only a general understanding of how they are used in the daily lives of amputees. To date, little is known about the types of activities amputees experience in daily life and how those activities influence overall ease with which an amputee is able to participate when using a specific prosthesis.
If real-world activity in the lower-extremity amputee population is to be measured and recorded, a system must be employed that is portable, unobtrusive, and is able to record activity over an extended period of time. Clearly, this requirement effectively eliminates using wires to couple monitoring sensors to a computer or other type of remote telemetry recorder. Furthermore, research is needed to accurately assess the data acquired in order to uniquely identify different types of activity. The objective of such research would be, for example, to identify, quantify, and characterize real-world ambulation in the trans-tibial amputee population. This assessment of and differentiation among real-world activities is critical to understanding prosthetic use and the functional characteristics needed from prosthetic components, devices, and systems. In addition, such a system can be very useful in assessing the proper fit of prosthetic devices to patients. Perhaps more importantly, such research has the potential to create a clinical tool with a wide scope of application—from clinical prescription of a device, to financial justification for an intervention, to the evaluation of a rehabilitation process.
Multiple Axes of Measurement Vs. Selected Axes of Measurement
Although commercial load sensing devices exist specifically for use in amputee prosthetic limbs, none simultaneously and reliably measure both force and moment. In particular, these devices do not measure force and moment well when force is applied in the forefoot, as commonly occurs during the late stance phase of gait. The problem occurs because of crosstalk problems between orthogonal axes of measurement, or between force and moment components of measurement. High bending moments in the sagittal plane tend to distort the force and other moment measures. As a result, interpretation of the force data is limited to conditions where bending moments are low (standing, mid-stance), or exclusively to force, exclusively to moment, or only to selected combinations of force and moment. This issue is relevant in certain application, for example to determine proper prosthetic alignment, since efforts in the 1990's demonstrated that sagittal and frontal bending moments did not predict proper alignment nearly as well as when other components were considered. Thus, a device is needed for use in amputee prosthetic limbs that measures forces and moments reliably across a wide range of loads, and which measures more or all components of force and moment.